This project deals with the structure and function of major transplantation antigens in rats. In particular it is concerned with the relationship between the immune response against major transplantation antigens and against conventional antigens. The relationship between the two is of major theoretical interest in view of conflicting ideas about the nature of T cell specificity and the origin of the T cell repertoire. Genetic control of the immune response to major transplantation antigens has suggested that the same antigenic molecules may function either as a 'major' antigen or as a 'conventional' antigen. We are investigating the relationship between these two functional attributes of the same molecule. The role of major transplantation antigens in regulation of immune responses is being investigated through the mode of action of immune response genes.